<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Document</title>
  </head>
  <body>
    <button id="btn">获取event对象</button>
    <script>
      var btn = document.getElementById('btn');
      btn.onclick = function (e) {
        var event = e || window.event;    // 获取事件对象的兼容处理
        console.log(event);
      };
    </script>
  </body>
</html>